Detector for detecting objects moving through a magnetic field established between coils of an l-c oscillator

ABSTRACT

A detector for detecting objects as they move through a magnetic field established between a transmitter coil connected to an electrical supply and a receiver coil spaced from and magnetically coupled to the transmitter coil. The receiver coil, together with a capacitor, form the oscillatory circuit of an amplifier to provide an oscillator which oscillates to provide a high amplitude output signal in response to alteration of the magnetic field due to an object passing therethrough. The oscillator is provided with three feedback networks including one feedback network which functions as a source of alternating current which causes the oscillator to provide the low amplitude output signal. The oscillatory circuit includes a resistor for providing a highly damped operation which supports the effect of the first mentioned feedback circuit. A second feedback circuit provides constant amplification and a third feedback circuit includes the transmitter and receiver coils for effecting the regenerative function for oscillation.

finite States Patent [191 Wentzel et al.

[m 3,72%2 Mar. 27, 1973 DETECTOR FOR DETECTING OBJECTS MOVING THROUGH AMAGNETIC FIELD ESTABLISHED FOREIGN PATENTS OR APPLICATIONS 1,298,5557/1969 Germany ..33l/65 BETWEEN COILS OF AN Primary Examiner-Robert J.Corcoran OSCILLATOR Attorney-Hill, Sherman, Meroni, Gross & Simpson [75]Inventors: Peter Wentzel; Edwald Schulze,

both of Munich, Germany [57] ABSTRACT [73] Assignee: SiemensAktiengesellschaft, Berlin A f" detecmIg objects as they move.through amagnetic field established between a transmitter (201] and Munich,Germany connected to an electrical supply and a receiver COlI [22]Filed: Apr. 5, 1971 spaced from and magnetically coupled to thetransmitter coil. The receiver coil, together with a capaci- [21] Appl'130381 tor, form the oscillatory circuit of an amplifier to provideanoscillator which oscillates to provide a high [30] Foreign Ap licationPriority Data am litude out ut signal in response to alteration of p h pp t" id d b t e magnetic le ue to an o ject passing Apr. 29, l970Germany ..P 20 21 102.3 therethrou h The oscillator is provided withthree fdbk k'ld' fdbk k ee ac networ s 1116 u mg one ee ac networ fjglgfi which functions as a source of alternating current which causes theOscillator to provide the low [58] Field at Search 3 plitude outputsignal. The oscillatory circuit includes a 11/ l resistor for providinga highly damped operation which supports the effect of the firstmentioned feed- [56] Reerences cued back circuit. A second feedbackcircuit provides constant amplification and a third feedback circuitiIl'lcludes the transmitter and receiver COlIS for effecting 2,505,5774/1950 7 Rich ..33l/65 the regenerative function for oscillation3,469,204 9/1969 Magyar et al.... .....33l/65 3,609,580 9/1971 Thompson..33 H65 6 Claims, 3 Drawing Figures AMPLITUDE LIMITING FEEDBACK CIRCUITTHRESHOLD f cmcun sw/ V f T" AMPLIFIER Ua m l- SLOT-TED DISC ll DETECTORFOR DETECTING OBJECTS MOVING THROUGH A MAGNETIC FIELD ESTABLISHEDBETWEEN COILS OF AN L-C OSCILLATOR BACKGROUND OF THE INVENTION 1 Fieldof the Invention This invention relates to apparatus for detectingmoving objects, and in particular to an L-C oscillator for accuratedetection of moving objects.

2. Description of the Prior Art The prior art includes a detector forobjects which when introduced into a magnetic field, change the magneticfield. A transmitter provides the source of the magnetic field and areceiver is coupled to the transmitter and spaced therefrom sufficientlyto provide passage of objects to be registered therebetween withoutphysical contact.

During the above process, the inductive coupling between the transmitterand the receiver is altered by passage therebetween of an object to beregistered which causes a voltage change in the receiver. This voltagechange can be sensed and directly or indirectly utilized in order tooperate electric switches or the like. In order to be able to providemeasurements which are as precise as possible, the object of Germanpatent application P 74 563.8 is to improve the accuracy andreproducibility of the signals received from such detectors.

According to the above-identified application, the accuracy andreproducibility is improved through the provision of magnet cores forestablishing the magnetic field, which cores are disposed in a magneticfield cabinet to provide fixed flux line concentration. The magnet coresare aligned and provided with relatively small frontal surfaces at thesides of the path of motion of the objects to be registered. A magneticscreening installation is provided for at least the transmitter or atleast the receiver to shield the corresponding core except for its smallfront surface.

In response to the changing electric voltage in the receiver of such anarrangement, electric switches or the like may be operated as mentionedabove. These signal emitters can comprise, for example, an L-Coscillator, the resonant circuit inductance of which is formed by thecoil of either the transmitter or the receiver. Since generally an'oscillator only starts to oscillate when a disturbance exists, the pointin time of the initiation of oscillation of a conventionalselfoscillating detector statistically varies according to the inherentnoise of the circuit. These fluctuations impair the time accuracy of thedetector.

SUMMARY OF THE INVENTION It is therefore the primary object of theinvention to further improve the time accuracy of a detector for movableobjects with an Ir-C oscillator as a signal emitter, which oscillatorhas resonant circuit whose characteristics are altered when an object tobe registered is guided past a receiver in a contactless manner. Thistechnique is also applicable to the inductive detector disclosed in theaforementioned German patent application.

According to the invention, the aboveobjective is realized through theprovision of an alternating voltage source which is connected to theinput of the oscillator in such a manner so that the oscillator operatesin a state of rest (during absence of an object) to provide alowamplitude signal and upon detection of an object it immediatelyoscillates to provide a high amplitude signal. With this technique, theproblem of statistical fluctuations at the point in time of oscillationinitiation in the prior art apparatus is overcome and accordingly, theaccuracy of the detector is not influenced. The advantages of thedetector of the above prior art detector circuit improved in accordancewith the present invention become truly evident.

A particularly advantageous further development of this inventionresides in the provision of the alternating v voltage source as anamplitude-dependent feedback .addition, the additional inverse feedbackcircuit improves detector accuracy in that it renders the amplificationof the oscillator substantially independent of the frequency.

BRIEF DESCRIPTION OF THE DRAWING Other objects, features and advantagesof the invention, its organization, construction and operation will bebest understood from the following detailed description of an exemplaryembodiment thereof taken in conjunction with the accompanying drawing,in which:

FIG. 1 is a schematic block diagram of a detector according to theinvention;

FIG. 2 is a circuit diagram corresponding to the circuit of FIG. 1; and

FIG. 3 is a schematic representation of a transmitter and receiver foran inductive detector having coils which form a feedback network of theoscillator.

DESCRIPTION OF THE PREFERRED EMBODIMENT The position of movable objectsis to be registered by means of a detector. In the particular exampleillustrated in the drawing, a rotating slotted disk SS is schematicallyillustrated in FIG. 1 to represent such objects. The disk SS hasradially extending slots distributed about its circumference, the frontand back edges of the slots representing the leading and trailing edgesof the objects being directed past a receiver element of the detector.The receiving element of the detector essentially comprises a receiver.coil L1. This coil L1 and a further coil L2, which is designed as anemitting coil, together form a feedback network of an L-C oscillator.Both coils L1, L2 are inductively coupled with each other and theslotted disk SS rotating therebetween alters the coupling factor of thecoils since the slotted disk SS has good electric conductance andstrongly suppresses the coupling as long as no slot of the disk istraversing the area between the emitting coil L2 and the receiver coilL1.

The receiver coil L1 and a capacitor C1 which is \nnected between thiscoil and the input of an oscillat together form the oscillatory circuitof the oscillator. The oscillator is designed in such a manner that itonly oscillates to provide a large amplitude oscillatory signal when aslot is located between the receiver coil L1 and the emitting ortransmitting coil L2, but since such an oscillator only starts tooscillate when a disturbance occurs, the point of time of initiation ofthe oscillation fluctuates statistically in accordance with theself-noise of the circuit. These fluctuations would impair the timeaccuracy of the detector to an unacceptable degree. In order to avoidthis situation, the oscillator is provided with a further feedbacknetwork RN which is constructed as an amplitude-limiting network.Therefore, it is provided that the oscillator oscillates continuously toprovide a small amplitude oscillatory signal. A change of the voltageinduced in the receiver coil L1 therefore only affects the amplitude ofthe oscillator delayed by the time constant involved in the initiationof oscillation.

Through the utilization of the above techniques, it now becomes evidentthat the oscillator has two oscillatory states according to whether ornot a slot is just moving past the receiving element. These two statesof oscillation are elevated by means of a threshold value circuit SWwhich is connected to the output of the oscillator, which thresholdvalue circuit SW is energized in response to the larger amplitudesignals to emit an output signal Ua.

The switching arrangement of the detector is illustrated in the circuitdiagram of FIG. 2. The oscillatory circuit of the oscillator is formedby the receiver coil LI and the capacitor C1 connected to the input ofan amplifier V. A resistor R2 is connected between the receiver coil L1and the capacitor C1 and is effective to suppress or damp theoscillations established in the coil L1 and the capacitor C1. With thiscircuit arrangement, the time constant for the initiation of oscillationof the oscillator can be kept to a small value; in addition, the qualityof the coil Ll has only a small influence. Attention is invited that theoscillatory circuit Ll, C1, R2 is operated with a 45 detuning in orderto obtain a phase rotation of the signal which is fed back by way of thecoils L2, L1. The amplifier V is constructed as a two-stage amplifierfrom a pair of transistors T1 and T2. In this particular case, the baseof the first transistor T1 is connected with the oscillatory circuitcapacitor C1, the emitter is grounded and the collector is connected tothe positive supply voltage +Ub by way of a collector resistor R3. Thebase of the second transistor T2 is connected to the collector of thefirst transistor T1, the emitter of the second transistor T2 isconnected to ground by way of a voltage divider circuit including a pairof resistors R5 and R6, and the emitter is further connected to the baseof the first transistor T1 by way of a resistor R4 which has a furtherresistor R9 connected in parallel therewith to serve as a compensatingresistance. This last-mentioned circuit forms an inverse feedbacknetwork which is proportioned in such a manner that the amplification ofthe entire oscillator remains substantially constant.

The oscillator also comprises, according to the present invention, afeedback network RN. This network is realized through the provision of aresistor R7 and a resistor R1 which are connected to each other andrespectively to the collector of the transistor T2 and to the junctionof the coil L1 and the resistor R2 at the input or base circuit of thetransistor T1. The network further includes a pair of parallel connectedoppositely poled diodes D1, D2 which are connected between the resistorsR1, R7 and the supply potential +Ub to provide an amplitude-dependentfeature for the network. By way of the proportioning of the feedbacknetwork RN, the alternating voltage connected to the limiter diodes D1,D2 is fed back over the resistor R1 and the oscillator circuit L1, R2,C1 in such a manner that the oscillator amplitude lies below the controlresolution of the threshold value circuit SW. In addition to this, theoscillator includes a further feedback network which comprises theemitting coil L2 positioned in the transmitter of the detector andconnected between the collector of the transistor T2 and the positivesupply potential +Ub, and the receiver coil Ll which represents theoscillatory circuit inductance and -which is positioned in the receivingelement of the detector. The values of the emitter coil L2 and of theemitter resistor R7 of the second amplifier stage cause a certain phaserotation of the feedback signal which, together with the detuning ofoscillatory circuit L1,C1, R2, is sufficient to meet the phaserequirement at the input of the amplifier V.

The physical arrangement of the emitting coil L2 and the receiver coilL1 is evident from FIG. 3 which schematically illustrates the emittingand receiving elements of an inductive detector. The emitter elementincludes the emitting coil L2 which has a magnetic core K2 and which issurrounded by a magnetic screening pot AT. On the surface opposite tothe receiving element, the magnetic core K2 of the emitting coil L2 andits screening pot AT are insulated from one another through theprovision of small insulating plates I. This construction of the emitteraccording to the theory of the aforementioned patent application rendersa strong concentration of the magnetic field between the emitting coilL2 and the receiving coil Ll coupled thereto. The receiving coil L1 hasa magnetic core K1 associated therewith. If, as in the selectedillustrative example, a rotating slotted disk SS is moved between theemitter and the receiver, the strongly concentrated magnetic field ischanged thereby. A slot guided between the emitter and the receiver doesnot influence the magnetic field and the coupling between the two coilsL1 and L2 is increased during this process. In the receiver coil L1, ahigher voltage is induced which is sufficient to enable the oscillatorto operate with a sufficient amplitude to overcome the threshold of thethreshold circuit SW in response to the passage of areas between theslots of the disk SS.

The threshold circuit SW comprises a further transistor T3 which isoperated in an emitter-base circuit configuration and has a baseresistor R8 connected to the junction point of the resistors R5, R6 ofthe emitter voltage divider associated with the transistor T2. In thestate of rest of the oscillator, i.e. as long as it oscillates with asmall amplitude, a direct voltage and a small superimposed alternatingcurrent are provided from the voltage divider network to the base of thetransistor T3, which signals are ineffective to operate the transistorT3; the transistor T3 remains securely blocked. In contrast to this, ifthe oscillator oscillates with sufficient amplitude, the alternatingvoltage at the resistor R6 is superimposed upon the direct currentpotential at the resistor R6 so that the transistor T3 is periodicallyenergized. In this case, the threshold value circuit SW emits a largeoutput signal Ua which is screened through a smoothing capacitor C2connected between the emitter and collector of the transistor T3, theemitter and the capacitor C2 being grounded.

Although the instant invention has been described above by means of aparticular exemplary embodiment thereof, it is not limited to thisparticular embodiment. Preferably, the invention is to be utilized forinductive detectors which are constructed according to the principlesset forth in the above-identified German application for patent, butthis is nevertheless a limitation on the invention. The invention isgenerally advantageous when utilized in the case of detectors formovable objects, if high accuracy of the position of the object to beregistered is of importance. In this case, such an object cannot only bedetected through the variable coupling between two coils; theoscillatory circuit characteristics could be changed, for example, bydamping to a stronger extent from a high amplitude signal to a lowamplitude signal in response to object detection, and also objectdetection may be determined by a change of capacitance of theoscillatory circuit.

Many other changes, modifications and alterations may be made of theinvention by one skilled in the art without departing from the spiritand scope of the invention, and it is to be understood that we intend toinclude within the patent warranted hereon all such changes,modifications and alterations which may reasonably and properly beincluded within the scope of our contribution to the art.

What we claim is:

1. Apparatus for detecting objects as they move through a magneticfield, comprising: an L-C oscillator having an input and an output atransmitter coil connected to an electrical supply and to the output ofsaid oscillator to produce a magnetic field; a receiver coil spaced fromand magneticallyv coupled to said transmitter coil, said objects passingthrough the space between said coils to alter the magnetic couplingtherebetween;-said oscillator input connected to said receiver coil,said receiving coil constituting the inductance of the oscillator L- Ccircuit, said oscillator providing an output signal of a first amplitudein response to alteration of the magnetic coupling between said coils,said oscillator including first and second transistors each having abase, an emitter and a collector, the collector of said first transistorconnected to the base of said second transistor, said base of said firsttransistor serving as said input of said oscillator and thecollector-emitter circuit of said second transistor serving as theoutput of said oscillator; an alternating voltage source connected tosaid oscillator causing said oscillator to produce an output signal of asecond amplitude when said magnetic coupling between said coils isunaltered, said alternating voltage source including anamplitude-dependent feedback circuit comprising a first resistorconnected to the base of said first transistor, a second resistorconnected between said first resistor and the collector of said secondtransistor and a pair of oppositely poled parallel connected diodesconnected between the junction of said first and second resistors andthe electrical supply for limiting the amplitude of the feedback signal;and a reversefeedback network including a third resistor connectedbetween said emitter of said second transistor and the base of saidfirst transistor for maintaining constant amplification for theamplifier portion of said oscillator.

2. Apparatus according to claim 1, comprising a further feedback networkwhich includes said transmitter coil connected between the electricalsupply and the collector of said second transistor, and said receivercoil connected between the base of said first transistor and a referencepotential.

3. Apparatus according to claim 2, comprising a capacitor connected incircuit with said receiver coil to form the L-C oscillating circuit ofsaid oscillator, and a fourth resistor connected in circuit with saidoscillatory .circuit to provide strong damping of the oscillationsthereof.

4. The apparatus according to claim 1, comprising a threshold circuitconnected to the output of said oscillator and operable to provide anamplitude indicative signal in response to an output signal of saidfirst amplitude.

5.-The apparatus according to claim 4, comprising a voltage dividercircuit connected between the emitter of said second transistor and areference potential, and said threshold circuit comprises a thirdtransistor having a base, an emitter'and a collector, a fourth resistorconnected between said voltage divider and the base of said thirdtransistor, and a smoothing capacitor connected between the collectorand emitter of said third transistor.

6. Apparatus for detecting objects as they move through a magneticfield, comprising: an LC oscillator capable of operating to provide anoutput signal of one amplitude and of another amplitude, said oscillatorhaving an input, an output, and an oscillatory circuit including acapacitor connected to said input and a first coil connected to saidcapacitor, a feedback circuit also including said first coil and asecond coil connected to an electrical supply and to the output of saidoscillator, said coils spaced apart and magnetically coupled, themagnetic coupling being altered by an object passing between said coilscausing said oscillator to oscillate to provide said output signal atsaid one magnitude, an AC amplitude-dependent feedback circuit connectedbetween the output and the input of said oscillator to cause saidoscillator to oscillate'to provide said output signal at said otheramplitude in the absence of alteration of the magnetic coupling betweensaid coils, and a reverse feedback circuit connected between said outputand said input of said oscillator for maintaining a constantamplification of said oscillator, said oscillatory circuit including aresistor connected to said capacitor and to said first coil for strongdamping of oscillations.

1. Apparatus for detecting objects as they move through a magneticfield, comprising: an L-C oscillator having an input and an output atransmitter coil connected to an electrical supply and to the output ofsaid oscillator to produce a magnetic field; a receiver coil spaced fromand magnetically coupled to said transmitter coil, said objects passingthrough the space between said coils to alter the magnetic couplingtherebetween; said oscillator input connected to said receiver coil,said receiving coil constituting the inductance of the oscillator L-Ccircuit, said oscillator providing an output signal of a first amplitudein response to alteration of the magnetic coupling between said coils,said oscillator including first and second transistors each having abase, an emitter and a collector, the collector of said first transistorconnected to the base of said second transistor, said base of said firsttransistor serving as said input of said oscillator and thecollector-emitter circuit of said second transistor serving as theoutput of said oscillator; an alternating voltage source connected tosaid oscillator causing said oscillator to produce an output signal of asecond amplitude when said magnetic coupling between said coils isunaltered, said alternating voltage source including anamplitude-dependent feedback circuit comprising a first resistorconnected to the base of said first transistor, a second resistorconnected between said first resistor and the collector of said secondtransistor and a pair of oppositely poled parallel connected diodesconnected between the junction of said first and second resistors andthe electrical supply for limiting the amplitude of the feedback signal;and a reverse feedback network including a third resistor connectedbetween said emitter of said second transistor and the base of saidfirst transistor for maintaining constant amplification for theamplifier portion of said oscillator.
 2. Apparatus according to claim 1,comprising a further feedback network which includes said transmittercoil connected between the electrical supply and the collector of saidsecond transistor, and said receiver coil connected between the base ofsaid first transistor and a reference potential.
 3. Apparatus accordingto claim 2, comprising a capacitor connected in circuit with saidreceiver coil to form the L-C oscillating circuit of said oscillator,and a fourth resistor connected in circuit with said oscillatory circuitto provide strong damping of the oscillations thereof.
 4. The apparatusaccording to claim 1, comprising a threshold circuit connected to theoutput of said oscillator and operable to provide an amplitudeindicative signal in response to an output signal of said firstamplitude.
 5. The apparatus according to claim 4, comprising a voltagedivider circuit connected between the emitter of said second transistorand a reference potential, and said threshold circuit comprises a thirdtransistor having a base, an emitter and a collector, a fourth resistorconnected between said voltage divider and the base of said thirdtransistor, and a smoothing capacitor connected between the collectorand emitter of sAid third transistor.
 6. Apparatus for detecting objectsas they move through a magnetic field, comprising: an L-C oscillatorcapable of operating to provide an output signal of one amplitude and ofanother amplitude, said oscillator having an input, an output, and anoscillatory circuit including a capacitor connected to said input and afirst coil connected to said capacitor, a feedback circuit alsoincluding said first coil and a second coil connected to an electricalsupply and to the output of said oscillator, said coils spaced apart andmagnetically coupled, the magnetic coupling being altered by an objectpassing between said coils causing said oscillator to oscillate toprovide said output signal at said one magnitude, an ACamplitude-dependent feedback circuit connected between the output andthe input of said oscillator to cause said oscillator to oscillate toprovide said output signal at said other amplitude in the absence ofalteration of the magnetic coupling between said coils, and a reversefeedback circuit connected between said output and said input of saidoscillator for maintaining a constant amplification of said oscillator,said oscillatory circuit including a resistor connected to saidcapacitor and to said first coil for strong damping of oscillations.